The present invention relates generally to the field of semiconductor wafer fabrication equipment useful in the manufacture of semiconductor devices, more particularly to chemical vapor deposition (CVD) apparatus useful for depositing a variety of different types of layers from a source gas onto a substrate, and especially to improvements resulting in a reduction of particulate contamination in such apparatus.
Apparatus of the above type is used in the fabrication of semiconductor devices on 100 millimeter or larger wafers of silicon. Epitaxial layers are formed on the surfaces of the wafers by heating them to temperatures in the region of 800 to 1200 degrees Celsius in a bell jar containing a gaseous atmosphere consisting of a hydrogen carrier gas mixed with one or more reactive gases such as a silicon source gas and a dopant source gas.
During the various processing steps which are used to produce layers of different types on the surfaces of the wafers, great care must be taken to avoid the deposition of particulate contaminants on the surfaces of the wafers. The demand for every larger scale of circuit integration and the consequent need for greater chip density have made the elimination of particulate contaminants an increasingly difficult problem. As the size of the circuit features has become smaller, the minimum size of particulate contaminants which can degrade circuit performance has also become smaller. This is true because particles as small as 1/10 the size of the circuit features can seriously degrade the electrical properties of the circuit.
One source of such contaminants is the exhaust line used to conduct gases from the interior of the bell jar when they are no longer needed. Such an exhaust line may terminate in a sump at atmospheric pressure or may connect the processing system to a source of high vacuum such as a vacuum pump. During normal processing this line is open to the vacuum pump, such that there is a steady flow of gases, including entrained particulate contaminants, in a direction away from the reaction chamber.
However, when a processing step has been completed, a valve in the exhaust line is closed while the reaction chamber is back-filled with a new processing atmosphere through an inlet line. During this back-filling operation, strong currents of moving gases in the reaction chamber can sweep up and entrain particulate contaminants from the region of the exhaust line near its junction with the reaction chamber.
The presence of contaminant particles in the short section of exhaust line between the valve and the reaction chamber is difficult to avoid, since some particulate matter is inevitably present in the effluent from the reaction chamber. However, the particle performance of the system could be improved if such particulate contaminants could be prevented, or at least inhibited, from returning to the reaction chamber during the back-filling operation.